The present invention relates to the field of semiconductor integrated circuits and, in particular, to metal-polysilicon contacts capable of tolerating high temperature oxidizing environments.
Semiconductor integrated circuits with high device density pose increasing difficulty to the formation of high-reliability electrical connections between metalization layers and semiconductor elements, particularly between the metal of a metallic electrode and the adjacent polysilicon of a polysilicon plug. This increased difficulty stems mainly from the tendency of metal and silicon to interdiffuse when in contact with each other, and when subjected to the high temperatures necessary during the fabrication of integrated circuits.
To illustrate the tendency of metal-silicon interdiffusion, the formation of a metallic contact between a polysilicon plug and a metallic electrode at a specified contact area will be briefly described bellow. FIG. 1 depicts a portion of a conventional memory cell construction for a DRAM at an intermediate stage of the fabrication, in which a metal-polysilicon contact is formed according to conventional processes.
A pair of memory cells having respective access transistors 33 are formed within a well 13 of a substrate 12. The wells and transistors are surrounded by a field oxide region 14 that provides isolation. N-type active regions 16 are provided in the doped p-type well 13 of substrate 12 (for NMOS transistors) and the pair of access transistors have respective gate stacks 30. The gate stacks 30 include an oxide layer 18, a conductive layer 20, such as poly silicon, nitride spacers 32, and a nitride cap 22. Additional stacks 31 may also be formed for use in performing self aligned contact etches to form conductive plugs for capacitor structures in the region between stacks 30,31. The details of these steps are well-known in the art and are not described in this application.
Next, a polysilicon plug 50 (FIG. 1) is formed in a contact opening of a first insulating layer 24, to directly connect to a source or drain region 16 of the semiconductor device. The first insulating layer 24 could be, for example, borophosphosilicate glass (BPSG), borosilicate glass (BSG), or phosphosilicate glass (PSG). Once the polysilicon plug 50 is formed, the whole structure, including the substrate 12 with the gate stacks 30, the first insulating layer 24 and the polysilicon plug 50, is chemically or mechanically polished to provide a planarized surface.
At this point, a second insulating layer 25, which can be of the same material as that of the first insulating layer 24, is deposited over the first insulating layer 24 and the polysilicon plug 50. A contact opening or via is etched over the polysilicon plug 50 and a metal layer or metal electrode 55 is then deposited and patterned to connect to the polysilicon plug 50, as illustrated in FIG. 1. Thus, polysilicon plug 50 comes into contact with the metal layer or electrode 55 at a metal-polysilicon interface 51 (FIG. 1). It must be understood, however, that, as known in the art, any other conductor, such as a capacitor plate for example, may also be in contact with a polysilicon plug, and the discussion herein applies to any metal-polysilicon interface.
Since several steps during the IC fabrication require temperatures higher than 500xc2x0 C., such as annealing steps, for example, silicon from the polysilicon plug 50 migrates into the metal film of the metallic electrode 55 during these high-temperature steps. Although this silicon migration into the metal film occurs in limited regions, near or at the metal-polysilicon interface 51, since the migrated silicon has high resistivity, the contact resistance at the metal-polysilicon interface 51 is greatly increased.
Barrier layers have been introduced to solve the silicon diffusion problem at the metal-polysilicon contact, such as interface 51 (FIG. 1). A barrier layer 52 is illustrated in FIG. 2 (which shows only a middle portion of the structure of FIG. 1). Conventionally, the barrier layer is a refractory metal compound such as refractory metal nitrides (for example TiN or HfN), refractory metal carbides (for example TiC or WC), or refractory metal borides (for example TiB or MoB). Barrier layers suppress the diffusion of the silicon and metal atoms at the polysilicon-metal interface, while offering a low resistivity and low contact resistance between the silicon and the barrier layer, and between the metal and the barrier layer. However, there is a problem with such barrier layers in that, in an O2 high temperature environment, they oxidize and disconnect the metal layer from the polysilicon plug. The oxide of the barrier layer may be formed either between the metal and the barrier layer, or between the polysilicon and the barrier layer. The latter situation is illustrated in FIG. 3, which shows metal oxide layer 53 formed between barrier layer 52 and polysilicon plug 50. In either case, the oxide of the barrier layer affects the conductive properties of the metal contact by increasing the electric resistance in the electrical connection region.
In an effort to reduce the oxidation problems posed by barrier layers subjected to oxidizing environments, different techniques have been introduced into the IC fabrication. One of them is manipulating and controlling the deposition parameters of the barrier materials. For example, U.S. Pat. No. 4,976,839 discloses that the presence of an oxide at grain boundaries within a titanium nitride (TiN) barrier layer improves the ability of the barrier layer to prevent the diffusion of silicon and aluminum. The reference further discloses a method for forming a barrier layer having large grain sizes by increasing the substrate temperature during sputtering, so that the formation of the oxide at the grain boundaries may be accomplished with a relatively large amount of oxygen, but without degradation in the film conductivity.
Similarly, to further improve the characteristics of the barrier layers, certain metals, for which both the oxidized species (MeO) as well as the unoxidized species (Me) are electrically conducting, have been recently used as barrier layers between metal and polysilicon. Examples of these metals are ruthenium (Ru), platinum (Pt), or iridium (Ir), among others. Since these barrier layers are conductive in both the metal and the oxide forms, this approach is useful in that both the oxide and the metal forms slow down the oxidation front in an O2 high temperature environment. However, this technique has a drawback in that there will still be some areas where the metal does not oxidize and, thus, the barrier layer would consist of portions of pure metal species and portions of metal oxide.
Accordingly, there is a need for an improved method for slowing down the oxidation front in barrier layers used in contacts between metal and polysilicon so that there is no oxidation at the polysilicon-metal interface. There is also a need for metal-polysilicon contacts that inhibit the diffusion of silicon and metal atoms at a contact interface and prevent the formation of oxides under high temperature O2 environment, as well as a method of forming such metal-polysilicon contacts.
The present invention provides a method for forming a metal-polysilicon contact that would be capable of tolerating an O2 environment up to several hundred degrees Celsius for several hours. To prevent a metal oxide front, which is formed during a high temperature O2 treatment from reaching the metal film at the metal-polysilicon interface, the metal film is surrounded by a plurality of oxygen sinks. These oxygen sinks are oxidized before the metal film at the bottom of the plug is oxidized. Accordingly, the conductive connection between the polysilicon and any device built on top of the barrier layer is preserved.
Additional advantages of the present invention will be more apparent from the detailed description and accompanying drawings, which illustrate preferred embodiments of the invention.